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Javascript
JavaScript is a scripting language used to enable programmatic access to objects within other applications. It is primarily used in the form of client-side JavaScript for the development of dynamic websites. JavaScript is a dialect of the ECMAScript standard and is characterized as a dynamic, weakly typed, prototype-based language with first-class functions. JavaScript was influenced by many languages and was designed to look like Java, but to be easier for non-programmers to work with. http:// JavaScript was originally developed by Brendan Eich of Netscape under the name Mocha, which was later renamed to LiveScript, and finally to JavaScript. The change of name from LiveScript to JavaScript roughly coincided with Netscape adding support for Java technology in its Netscape Navigator web browser. JavaScript was first introduced and deployed in the Netscape 2.0B3 in December 1995. The naming has caused confusion, giving the impression that the language is a spin-off of Java, and it has been characterized by many as a marketing ploy by Netscape to give JavaScript the cachet of what was then the hot new web-programming language. JavaScript, despite the name, is essentially unrelated to the Java programming language even though the two do have superficial similarities. Both languages use syntaxes influenced by that of C syntax, and JavaScript copies many Java names and naming conventions. The language's name is the result of a co-marketing deal between Netscape and Sun, in exchange for Netscape bundling Sun's Java runtime with their then-dominant browser. The key design principles within JavaScript are inherited from the Self and Scheme programming languages. "JavaScript" is a trademark of Sun Microsystems. It was used under license for technology invented and implemented by Netscape and current entities such as the Mozilla Foundation. Due to the widespread success of JavaScript as a client-side scripting language for web pages, Microsoft developed a compatible dialect of the language, naming it JScript to avoid issues. JScript added new date methods to fix the non-Y2K-friendly methods in JavaScript, which were based on java.util.Date. JScript was included in Internet Explorer 3.0, released in August 1996. The dialects are perceived to be so similar that the terms "JavaScript" and "JScript" are often used interchangeably. Microsoft, however, notes dozens of ways in which JScript is not ECMA-compliant. Netscape submitted JavaScript to Ecma International for standardization resulting in the standardized version named ECMAScript. JavaScript has become one of the most popular programming languages on the web. Initially, however, many professional programmers denigrated the language because its target audience was web authors and other such "amateurs", among other reasons. The advent of AJAX returned JavaScript to the spotlight and brought more professional programming attention. The result was a proliferation of comprehensive frameworks and libraries, improved JavaScript programming practices, and increased usage of JavaScript outside of the browser, as seen by the proliferation of Server-side JavaScript platforms. http:// The following features are common to all conforming ECMAScript implementations, unless explicitly specified otherwise. http:// JavaScript supports all the structured programming syntax in C (e.g., statements, loops, statements, etc.). One partial exception is scoping: C-style block-level scoping is not supported. JavaScript 1.7, however, supports block-level scoping with the keyword. Like C, JavaScript makes a distinction between expressions and statements. http:// http:// http:// http:// http:// JavaScript is officially managed by Mozilla, and new language features are added periodically. However, only some non-Mozilla "JavaScript" engines support these new features: *conditional clauses *property getter and setter functions *iterator protocol adopted from Python *shallow generators/coroutines also adopted from Python *array comprehensions and generator expressions also adopted from Python *proper block scope via new keyword *array and object destructuring (limited form of pattern matching) *concise function expressions () *E4X http:// As of 2008, the latest version of the language is JavaScript 1.8. It is a superset of ECMAScript (ECMA-262) Edition 3. Extensions to the language, including partial E4X (ECMA-357) support and experimental features considered for inclusion into ECMAScript Edition 4, are documented here. Sample code showcasing various JavaScript features:The output is: LCMCalculator: a = 28, b = 56, gcd = 28, lcm = 56 LCMCalculator: a = 21, b = 56, gcd = 7, lcm = 168 LCMCalculator: a = 25, b = 55, gcd = 5, lcm = 275 LCMCalculator: a = 22, b = 58, gcd = 2, lcm = 638 http:// The primary use of JavaScript is to write functions that are embedded in or included from HTML pages and interact with the Document Object Model (DOM) of the page. Some simple examples of this usage are: *Opening or popping up a new window with programmatic control over the size, position, and attributes of the new window (i.e. whether the menus, toolbars, etc. are visible). *Validation of web form input values to make sure that they will be accepted before they are submitted to the server. *Changing images as the mouse cursor moves over them: This effect is often used to draw the user's attention to important links displayed as graphical elements. Because JavaScript code can run locally in a user's browser (rather than on a remote server) it can respond to user actions quickly, making an application feel more responsive. Furthermore, JavaScript code can detect user actions which HTML alone cannot, such as individual keystrokes. Applications such as Gmail take advantage of this: much of the user-interface logic is written in JavaScript, and JavaScript dispatches requests for information (such as the content of an e-mail message) to the server. The wider trend of Ajax programming similarly exploits this strength. A JavaScript engine (also known as JavaScript interpreter or JavaScript implementation) is an interpreter that interprets JavaScript source code and executes the script accordingly. The first ever JavaScript engine was created by Brendan Eich at Netscape Communications Corporation, for the Netscape Navigator web browser. The engine, code-named SpiderMonkey, is implemented in C. It has since been updated (in JavaScript 1.5) to conform to ECMA-262 Edition 3. The Rhino engine, created primarily by Norris Boyd (formerly of Netscape; now at Google) is a JavaScript implementation in Java. Rhino, like SpiderMonkey, is ECMA-262 Edition 3 compliant. A web browser is by far the most common host environment for JavaScript. Web browsers typically use the public API to create "host objects" responsible for reflecting the DOM into JavaScript. The web server is another common application of the engine. A JavaScript webserver would expose host objects representing an HTTP request and response objects, which a JavaScript program could then manipulate to dynamically generate web pages. A minimal example of a standards-conforming web page containing JavaScript (using HTML 4.01 syntax) would be:http:// The DOM interfaces for manipulating web pages are not part of the ECMAScript standard, or of JavaScript itself. Officially, they are defined by a separate standardization effort by the W3C; in practice, browser implementations differ from the standards and from each other, and not all browsers execute JavaScript. To deal with these differences, JavaScript authors can attempt to write standards-compliant code which will also be executed correctly by most browsers; failing that, they can write code that checks for the presence of certain browser features and behaves differently if they are not available. In some cases, two browsers may both implement a feature but with different behavior, and authors may find it practical to detect what browser is running and change their script's behavior to match. Programmers may also use libraries or toolkits which take browser differences into account. Furthermore, scripts will not work for all users. For example, a user may: *use an old or rare browser with incomplete or unusual DOM support, *use a PDA or mobile phone browser which cannot execute JavaScript, *have JavaScript execution disabled as a security precaution, *or be visually or otherwise disabled and use a speech browser To support these users, web authors can try to create pages which degrade gracefully on user agents (browsers) which do not support the page's JavaScript. http:// JavaScript and the DOM provide the potential for malicious authors to deliver scripts to run on a client computer via the web. Browser authors contain this risk using two restrictions. First, scripts run in a sandbox in which they can only perform web-related actions, not general-purpose programming tasks like creating files. Second, scripts are constrained by the same origin policy: scripts from one web site do not have access to information such as usernames, passwords, or cookies sent to another site. Most JavaScript-related security bugs are breaches of either the same origin policy or the sandbox. http:// A common JavaScript-related security problem is cross-site scripting, or XSS, a violation of the same-origin policy. XSS vulnerabilities occur when an attacker is able to cause a target web site, such as an online banking website, to include a malicious script in the webpage presented to a victim. The script in this example can then access the banking application with the privileges of the victim, potentially disclosing secret information or transferring money without the victim's authorization. A solution to XSS vulnerabilities is to use HTML escaping whenever displaying untrusted data. XSS vulnerabilities can also occur because of implementation mistakes by browser authors. Another cross-site vulnerability is cross-site request forgery or CSRF. In CSRF, code on an attacker's site tricks the victim's browser into taking actions the user didn't intend at a target site (like transferring money at a bank). It works because, if the target site relies only on cookies to authenticate requests, then requests initiated by code on the attacker's site will carry the same legitimate login credentials as requests initiated by the user. In general, the solution to CSRF is to require an authentication value in a hidden form field, and not only in the cookies, to authenticate any request that might have lasting effects. Checking the HTTP Referrer header can also help. "JavaScript hijacking" is a type of CSRF attack in which a